JP-A 7-139397 discloses an internal combustion engine having an exhaust gas passageway in which a NOx absorbent is disposed. When an air-fuel ratio of exhaust gas upstream of the absorbent is greater than the stoichiometry (A/F), the absorbent absorbs NOx in the exhaust gas. When STOICH the air-fuel ratio is not greater than the stoichiometry, the absorbent releases NOx.
FIG. 13 illustrates schematically a target A/F command input into a closed fuel control loop of an internal combustion engine having a NOx absorbent catalyst and NOx emission in the exhaust gas upstream of the catalyst. It also illustrates an air-fuel ratio in the exhaust gas, namely, an exhaust gas air-fuel ratio, at an inlet of the converter (drawn by the fully drawn line) and an exhaust gas air-fuel ratio at an outlet of the converter (drawn by the broken line). In FIG. 13, the fully drawn curve shows NOx emission resulting from combustion during lean combustion mode. The NOx absorbent has a storage limit. If this storage limit is exceeded, NOx is not absorbed.
Usually, the engine load, engine speed, and air-fuel ratio are employed for a controller to determine whether or not the storage limit is accomplished. If the storage limit is about to be exceeded, the controller sets the target air-fuel ratio to the stoichiometry or rich side, i.e., A/F.ltoreq.A/F.sub.STOICH, causing the absorbent to release NOx, subjecting the released NOx to reduction with HC and CO contained in the exhaust gas.
A change, in target air-fuel ratio, from the lean side to the stoichiometry or rich side causes separation of NOx and O.sub.2 from the NOx absorbent, restraining the atmosphere within the absorbent from becoming rich in air-fuel ratio due to the presence of separated O.sub.2. The overabundance of O.sub.2 oxidizes HC and CO, within the absorbent, which serve to purify NOx. Thus, the separated NOx is not purified at a satisfactory rate and discharged as it is out of the catalytic converter.
For purification of NOx, it would be desirable to accelerate the rate at which the exhaust gas within the converter becomes rich by giving rich spike in air-fuel ratio of the combustible charge.
The maximum amount by which a NOx absorbent can absorb NOx and O.sub.2 storage capacity of the absorbent gradually decrease over extended period of time, thus, it would pose a problem of increased emission of HC and CO if the amount of rich spike were left unaltered cover the whole life of the NOx absorbent.
Besides, appropriately adjusting, in amount, rich spike proves to be an insufficient solution to the problem that NOx may leave the absorbent unpurified. This problem cannot be solved by increased amount of rich spike.
An object of the present invention is to provide a system for controlling air-fuel ratio of combustible charge, creating environment optimum for purification process of NOx in an exhaust passageway, which has a catalytic converter that carries a NOx absorbent, over the whole life of the NOx absorbent.